Many integrated circuits use thermistors for temperature sensing. For example, many rechargeable batteries include a thermistor in the battery pack, and the chargers use this thermistor to monitor the battery temperature as the battery charges. Control circuitry will disable the battery from charging when the temperature is either too hot or too cold for safe charging of the battery. For many applications, it is desirable to be able to reliably distinguish between the high resistance of a negative temperature coefficient thermistor during a cold condition from the very high or infinite resistance caused by an absent thermistor, which indicates that the battery is removed from the charger.
An example of such a sensor circuit will be described with reference to FIG. 1. The sensor circuit 100 includes a current source 110 that is connected to a variable resistor 120. The variable resistor 120 is of a thermistor type such that the resistance of the variable resistor 120 varies with the temperature of the resistor. Accordingly, the voltage at node 125 will vary with the resistance of the variable resistor 120. Comparators 130 and 140 are in communication with the node 125 to sense the voltage at the node 125. Comparator 130 compares the voltage at node 125 with the reference voltage VR1 and outputs a signal depending on whether the voltage at node 125 is higher than or less than the reference voltage VR1. Similarly, comparator 140 compares the voltage at node 125 with the second reference voltage VR2 and outputs a signal based upon whether the voltage at node 125 is higher than or less than the reference voltage VR2. Based upon the output of the comparators 130 and 140, an additional logic element 150 outputs a signal to the system indicating whether the temperature is too low to charge the battery or whether the battery has been removed from the charger.
An example of the operation of such a system will be described with reference to FIG. 2. In this example, the resistance of the variable resistor 120 is plotted against the temperature of the resistor in graph 200. As the temperature for the variable resistor 120 decreases, the resistance, in turn, increases. Accordingly, as shown in graph 205, the voltage of the node 125 also increases given the constant current applied by the current source 110. When the temperature is warm enough to be safe to charge the battery, the voltage at node 125 is below both of the reference voltages VR1 and VR2 such that neither comparator circuit 130 or 140 outputs a signal indicating a cold or open circuit. As the temperature decreases, approaching the temperature at which it is unsafe to charge the battery, the variable resistor 120 resistance begins to quickly increase thereby increasing the voltage. Eventually, the voltage at node 125 reaches and passes the reference voltage VR1 at which point the comparator 130 provides the signals to the system indicating that the temperature is too low such that the system stops charging the battery. At such low temperatures, the resistance of a typical thermistor increases rapidly in a non-linear fashion; the second reference voltage VR2, however, cannot be set higher than the system voltage VAA. Because of the non-linear relationship between the resistance of the variable resistor 120 and the temperature, the voltage at node 125 quickly exceeds the second reference voltage VR2 with the falling temperature such that comparator 140 provides a signal to the system indicating that the battery has been removed from the charger. Such close proximity between the resistor temperatures at which the node 125 reaches the reference voltages can often lead to erroneous readings that the battery has been removed from the system.